This invention relates to a circuit arrangement for driving a saw-tooth current in a coil, and is particularly applicable to the line output stage for providing a saw-tooth current in the horizontal deflection coils of a color television picture tube.
There is known, for example, from U.S. Pat. No. 4,161,675, a line output stage which comprises a trace capacitor and a retrace capacitor connected to a deflection coil to form a circuit which oscillates freely during the retrace interval, a first diode (termed the dumper diode) connected in parallel with the oscillating circuit to conduct during a first part of the trace interval, a second diode and a controllable semi-conductor switch connected in series with one another in a path parallel to the oscillating circuit. The second diode is conductive during a second part of the trace interval and the controllable switch is made conductive during part of the trace interval. The switch is also connected to a supply source via a first inductor so that during the part of the trace interval in which the switch is conductive energy is stored in the first inductor, at least part of which energy is later transferred via a third diode to the oscillating circuit during the period in which the switch is not conducting.
The self-stabilizing deflection circuit in a television set, usually referred to as a Wessel circuit, is a known circuit of the above type. An essentially similar circuit, known as a "POPSY" or a "Power Package System" has recently been proposed by the Philips company for stabilizing the supply of color television sets and generating high voltages for the picture tube.
By means of such a circuit, a deflection circuit on a television set can be supplied with a stabilized voltage lower than that available using normal power mains voltage rectifying circuits. However, the stabilized voltage obtained has a lower limit of about 150-160 V in the case of a mains supply of 220 V a.c. This figure takes into account the need to allow for a 10% increase and is higher for a mains supply of greater than 220 V a.c.
Accordingly, it is an object of the present invention to provide a circuit which allows the stabilized voltage to be reduced below the levels currently obtainable using known circuits.